Mortgage Servicing Rights - Sensitivity to Changes in Interest Rates of the Fair Value of MSR Portfolio and Related Derivative Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Down Scenario [Member] | Mortgage Servicing Rights [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	$ (619)	$ (442)
Net fair value 50 basis points	(320)	(271)
Net fair value 25 basis points	(161)	(150)
Down Scenario [Member] | Derivative [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	574	523
Net fair value 50 basis points	290	281
Net fair value 25 basis points	144	145
Down Scenario [Member] | Net Sensitivity [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	(45)	81
Net fair value 50 basis points	(30)	10
Net fair value 25 basis points	(17)	(5)
Up Scenario [Member] | Mortgage Servicing Rights [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	520	671
Net fair value 50 basis points	294	343
Net fair value 25 basis points	154	169
Up Scenario [Member] | Derivative [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	(524)	(625)
Net fair value 50 basis points	(270)	(304)
Net fair value 25 basis points	(137)	(149)
Up Scenario [Member] | Net Sensitivity [Member]
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Net fair value 100 basis points	(4)	46
Net fair value 50 basis points	24	39
Net fair value 25 basis points	$ 17	$ 20